Failures in asphalt concrete and cement pavements can result from instability, disintegration of the material, and cracking. Generally, instability and disintegration are results of faulty mix design and/or faulty construction procedures. Cracking, on the other hand, results primarily from thermal and traffic stresses which exceed the breaking strength of the road construction material.
Generally, thermal stresses are generated by changes in atmospheric temperatures which can cause rapid changes in temperature throughout the thickness of the pavement. Such rapid temperature changes, coupled with the fact that the pavement is insufficiently restrained from freely contracting, or expanding, results in stressing of the pavement. These stresses may accumulate to the extent that the strength of the material is exceeded, at which time visible cracking may form. High stresses due to temperature change can be reached in short periods of time, for example, only a few hours. For example, when a road cools rapidly at night, e.g. on the order of 6.degree. F. per hour, the thermal stress in the road can increase from about 10 pounds per square inch at freezing (32.degree. F.) to a value of about 220 pounds per square inch at -18.degree. F.
The additional stressing of the road caused by traffic loading also causes the road to deteriorate. However, in the case of traffic loading, stressing of the pavement occurs at even a much faster rate, typically in milliseconds.
In the repair of deteriorated asphalt concrete roads, one of the most persistent and troublesome problems is the phenomenon of "reflective cracking". This occurs where a new asphalt concrete surface, or "overlay", is applied to an old, cracked road surface, optionally, but typically after filling the cracks with a suitable asphaltic crack filler. After a relatively short period of time, the cracks from the old road will appear in the overlay, above the location of the old cracks. This is known as "reflective cracking," and it occurs because the old cracks propagate by widening or lengthening with sufficient force to tear the new overlay.
It has been determined that merely increasing the overlay thickness does not reduce the overall strain energy release rate or "stress intensity factor" and, thus, does not prevent reflective cracking. For example, increasing the overlay thickness from 1 inch to 3 inches will reduce the thermal stress intensity factor by about 17%, but it may actually increase the traffic stress intensity factor by as much as 50%. Thus, increasing the overlay thickness is relatively immaterial so far as thermal stress is concerned, and the thickness increase actually may be counterproductive because of increasing the traffic stress.
Reflective cracking is due primarily to these same thermal and traffic stresses which occur in the road, particularly those thermal stresses which are generated by the road cooling quickly. Where the thermally stressed road is also subject to traffic loads, crack propagation and reflective cracking is greatly increased, with the resultant rapid deterioration of the newly-applied overlay. Also, any intrusion of water through the reflective cracks will cause further deterioration of the entire road.
Attempts have been made to deal with the reflective cracking problem by providing membranes over the repaired crack prior to the application of the overlay. That is, the cracks were, first of all, filled or sealed with a suitable crack filler and then a reinforcing membrane was formed which spanned the crack. These membranes included a permeable reinforcing mat, typically a fibrous reinforcing mat which could be of glass or organic material such as, for example, polypropylene or other strong fibrous material like the polyesters, for example, PET. In one application technique an asphaltic based binder was applied about the crack, a fibrous glass woven roving was then applied onto the molten binder prior to its solidification and then the woven roving was overcoated with the same binder. In another technique a membrane was employed which included a reinforcing member which had been impregnated with an asphaltic based composition and then one side of that impregnated reinforcing member was coated with an adhesive which was pressure sensitive at ambient temperatures. This membrane was applied, or adhered, to the underlying distressed or cracked pavement by attaching the adhesive portion thereto and then the repair maintenance of the road was completed by overlaying with an asphaltic concrete.
While such approaches have provided improvement, nonetheless reflective cracking remains a problem in the art and there is still a need in the art to provide even more improvement.